1. Field of the Invention
The present invention relates in general to dry phosphate refractory concrete materials having MgO and AI(H.sub.2 PO.sub.4).sub.3, and more particularly, to special compositions which when synthesized yield nominally exothermic reactions, and use virtually "catalytic" amounts of active materials without sacrificing either structural integrity or variable setting times.
2. Background Art
Refractory concretes, also known as castables, are normally bonded with high-temperature calcium aluminate cement. Cement adlevels commonly range from one to forty percent and setting times are typically variable and range from 30 minutes to over 3 hours. In some instances, a fast setting time is desired, for example, when specialized manufacturing of uniquely-shaped burner block is desired, or, when rapid furnace repairs or patches are needed. Inasmuch as many thousands of dollars per hour are lost while a furnace is non-operational, minimizing such furnace down time is essential. Another example of when a fast set of the refractory material is desired is during the forming and pouring of furnace walls when construction time is extremely limited due to scheduling demands. Indeed, while accelerating the setting time of calcium aluminate concretes is known in the art, the ultimate structural integrity of the material does become adversely affected. Additionally, the initial dry-out and heat-up of the calcium aluminate concrete castable takes a substantial amount of time regardless of, and in addition to, the initial setting time of the mixture.
In addition to the above, safety must be considered when configuring a furnace heat-up schedule. For example, refractory calcium aluminate cement develops strength after hydrating the chemical reagents. Sufficient water must be charged to a cement-bonded high-temperature concrete to hydrate the cement and allow for placement and/or movement of the mass. After the cement-bonded concrete is hardened, the water must be removed slowly before the furnace can be put back into service. Consideration must be given to the permeability of the mass, dynamics of the cement phases and its hydration level. The end result is that heating rates for concrete cure can require up to several hundred hours to reach the furnace operating temperature. As the concrete is heated, the mass functions as a "leaky" autoclave. The pressure caused by the vaporization of the free water and steam released from the dehydration of the cement can be explosive, if the pressure exceeds the tensile strength of the castable. Even if the mass does not actually explode, rapid heating can cause internal cracking and damage that will shorten the ultimate life of the concrete material. This damage is known as thermal shock damage.
The long turn-around times for concrete furnace linings and possible thermal shock damage are just part of the problem associated with conventional refractory material. Indeed, if the furnace lining comes into contact with molten metal, an adverse chemical reaction can occur. This adverse reaction, as observed in calcium aluminate cement systems, is considered a weak link in the ability of refractory concretes to resist molten metal attack and/or penetration of the furnace lining.
Phosphate refractory concretes, on the other hand, have several advantages over traditional calcium aluminate cement-bonded products. The first benefit is that the phosphate bond is not affected by molten aluminum. The metal is non-reactive with the phosphate, unlike the calcium aluminate of traditional cements. Another benefit is curing or firing time. Phosphate-bonded materials generally can be heated much faster than traditional cement-bonded products. Furthermore, there is a much lower chance for sustaining thermal shock damage. Phosphate-bonded concretes use many different types of phosphates and often have a basic component such as magnesium oxide (MgO) which reacts with the phosphate in the presence of water (or an aqueous liquid) whereupon hardening occurs.
Although such conventional phosphate bonded concretes have exhibited various benefits over other conventional refractory materials, problems nevertheless persist. For example, when phosphate-bonded concretes are used, the reaction rate is often so fast that the concrete cannot be poured into place before it hardens. Additionally, when a liquid phosphate or phosphoric acid is used, safe handling of the toxic liquid presents a real hazard, not to mention the burden associated with working with a two-phase system.
Greger, U.S. Pat. No. 2,450,952 (hereinafter "Greger '952") appears to disclose a dry phosphate cement mixture for adhesive applications. The reagents used in Greger '952 included magnesia, olivine and or magnesium silicate mixed with water soluble aluminum phosphate. The weight ratio of the magnesium compound to the phosphate is disclosed to be 2:1 to 8:1. Inasmuch as the set is relatively fast when magnesia is used as a reagent, Greger '952, discloses substituting olivine for the magnesia, to, in turn, slow the set time for as much as 24 hours. However, olivine has limited high temperature applications due to melting point considerations and chemical reactivity at high temperature.
Tomic, U.S. Pat. No. 4,392,174 (hereinafter "Tomic '174") appears to disclose a mixture of magnesium oxide in aluminum phosphates, as well as using aluminum phosphates in liquid form. Aggregates, such as gravel or trap rock are combined with a mixture of magnesium oxide and phosphate, and then used for such applications as patching of highways. However, Tomic '174 teaches the use of high magnesium oxide concentration (as well as other high reagent concentrations). Although such a high concentration appears to provide a phosphate cement with great structural integrity, the percent composition of the active reagents is undesirably high. The result of having such high concentrations of active reagents is that undesirable levels of heat are generated as a result of the exothermic nature of the chemical reaction. Furthermore, the cost of the active reagents in phosphate concretes are quite expensive when compared to the cost of the inactive reagents. When used in such great concentrations, as taught in Tomic '174, the profitability of an installation is adversely affected.
It is thus an object of the present invention to provide a dry phosphate refractory concrete which can be synthesized in a cost effective manner.
It is a further object of the present invention to provide chemical compositions, such that when synthesized, liberate nominally exothermic properties.
It is yet a further object of the present invention to provide phosphate concretes as described above, without sacrificing structural integrity or the necessary enhancement of variable setting times.
More particularly, it is an object of the present invention that regardless of the specific active reagent concentrations (such as those experimentally identified in the present disclosure, relative to the present invention), other reagent concentrations less than conventionally known, and, which, in such relatively low concentration result in hardened refractory material maintaining excellent structural characteristics, are likewise fundamental to the objective parameters of the present invention.